1. Field of the Invention
This invention relates to scanning radio receivers and, more particularly, to scanning radio receivers particularly useful on the frequencies assigned to the Public Safety Radio Services as well as other frequencies.
2. Description of the Prior Art
Scanning radio receivers are well known in the prior art and have found particular utility for the reception of radio signals on the frequencies assigned by the United States Federal Communications Commission to the Public Safety Radio Services. In the past, many such radio receivers used crystals as the tuning element to provide the necessary local oscillator signals and required the presence of one crystal for each frequency the receiver was capable of tuning. Examples of such receivers are those shown in U.S. Pat. No. 3,531,724 to G. H. Fathauer, U.S. Pat. No. 3,665318 to S. J. Hoffman, et al., U.S. Pat. No. 3,714,585 to R. C. Koch, U.S. Pat. No. 3,725,788 to G. H. Fathauer, U.S. Pat. No. 3,794,925 to K. Imazeki, U.S. Pat. No. 3,801,914 to K. Imazeki, U.S. Pat. No. 3,821,651 to G. H. Fathauer, et al., U.S. Pat. No. 3,873,924 to G. H. Fathauer, U.S. Pat. No. 3,883,808 to J. E. Boone, U.S. Pat. No. 3,824,475 to P. W. Pflasterer, and U.S. Pat. No. 3,987,400 to G. H. Fathauer. There has also been provided by the prior art scanning radio receivers using frequency synthesizing techniques wherein the frequency synthesizing circuitry was controlled by the operation of a processing means such as a microprocessor. Exemplary radio receivers of this type are disclosed in U.S. Pat. Nos. 3,962,644 and 4,092,594, both to W. Baker.
Scanning radio receivers making use of microprocessors to control the frequency synthesizing circuitry have many advantages over the scanning radio receivers known prior thereto. One of the foremost of these advantages is the very high degree of flexibility in control which may be included in such a radio. An example is the "search" mode of operation shown in the aforementioned U.S. Pat. Nos. 3,962,644 and 4,092,594 by which the radio may be successively and automatically tuned to adjacent ones of the Public Safety Radio Service frequencies until the receiver arrives at a frequency upon which a signal is received. Such operation was impractical with scanning radio receivers known prior thereto.
The search mode of operation described above, while being a very advantageous feature, has certain disadvantages and problems which had not been recognized and attacked and, as a result, improved systems have been developed. In particular, systems which embody certain improvements are disclosed in U.S. Pat. Nos. 4,270,217, and 4,409,688, of which the applicant is inventor and U.S. Pat. No. 4,521,915 of which the applicant is co-inventor. As disclosed in U.S. Pat. No. 4,409,688, circuitry is provided for reducing the time required to shift from one frequency to another in a searching or scanning operation. The features to which the claims of the aforesaid application are directed are thus highly advantageous to a receiver incorporating a search mode but they did not address certain other problems associated with rapid frequency scanning.
In particular, the time to search or scan any particular frequency in a receiver using a phase-locked-loop frequency synthesizer circuit was at least as long as the sum of three time periods: the time required for the controller or microprocessor to generate the control signals necessary to tune the receiver to the new frequency, the "settling time" for the phase-locked-loop to adjust itself and bring the local oscillator signal to the desired frequency, and the time needed for the receiver circuitry to reliably detect the presence or absence of a received signal above a minimum signal level at the new frequency. Any steps taken to reduce any of these three time periods can be of importance in materially shortening the time required to search a large number of frequencies. The reduction occurs each time a new frequency is tuned so that the shortening of the time required to search a number of frequencies is equal to the reduction in the time period required for any individual frequency multiplied by the number of frequencies searched. In a receiver constructed in accordance with U.S. Pat. No. 4,409,688, a reduction in the time required to scan a particular frequency is achieved by overlapping the generation of the synthesizer control signals for the next frequency to be scanned with the phase-locked-loop settling time and the time required to detect the presence or absence of a received signal. The processor outputs a frequency code from a register and simultaneously synchronizes a timer and initiates the processing of data to supply a new code to the register. At the end of the time interval established by the timer, an interrupt signal is developed and if no indication is developed on a "signal present" line of the receiver the new code is immediately output to the synthesizer.
Any such additional reduction in time can be particularly advantageous in receivers such as the aforementioned which have the capability of searching all frequencies assigned to a particular service since in some cases, the number of frequencies assigned to a single service may be large. However, in shortening the time required to tune a receiver to a new frequency during the search or scan modes, it must be recognized that in some cases the user will use the receiver in a manual mode wherein he will either cause it to become tuned to either one of a plurality of preselected frequencies or to a frequency being newly specified by the user. Increasing the search or scan speeds should not be at the expense of eliminating the capability for operation of the receiver in the manual mode. Moreover, some scanning radio receivers include a priority operation wherein one receiver channel is assigned a priority status and the receiver automatically and periodically checks the frequency of that channel for the presence of a received signal and, if found, places the receiver on that channel. Any effort to shorten the scan or search time should not adversely affect the priority operation.